Hitherto chain scrapers have been used in many settling basins in order to remove the sludge which precipitate to the bottom of the basin. These chain scrapers consist of one or more scraping blades extending across the bottom of the basin and which are dragged forward by means of continuously going chains in a pater noster conveyer movement. These constructions have certain advantages by providing a good cleaning result and have moreover good functionality as long as the construction is in good order. However, in spite of its genuine basic construction serious casualties often occur. These depend on the heavy corrosion which all components, including the chains, are subject to. In the settling basins of the water purification plants the environment is very corrosive due to the acid aluminium and iron salts used in the chemical precipitation step present in the purification plants, which salts are used inter alia for the precipitation of phosphorous.
It has also been proposed a traverse which is driven above the water surface of the basin. A vertically rising scraper is mounted onto arms on the traverse. A drawback of this construction is that it is complicated to use in basins present, which quite often are provided with different equipment above the basin, such as walking bridges, and other equipment providing a hindrance. Further the settling is disturbed by the vertical devices holding the scraper.
Further, it has been proposed wire pulled scrapers or scraping devices where one or more wires pulls a carriage or sleigh holding scrapers on or close to the .bottom of the basin in a to and fro going movement. (SE-B-423,968; U.S. Pat. No. 2,866,557;4 U.S. Pat. No. 2,768,749; U.S. Pat. No. 4,090,966; GB-A-1,504,518). The technique is an improvement over prior art chain driven scrapers by a considerably more simple, lighter and cheaper construction. However, the system means that different devices have to be used in order to turn the scraping blade and to control this turning, from a vertical to a horizontal position and vice verse, during the reciprocable movement carried out by the wire/-s. The scraping blades, which consist of metal blades being edge mounted in the transporting direction and most often being provided with a rubber edge strip, extend across the width of the basin, perpendicular to the transporting direction. At the reciprocable movement of the scraping blades, sludge is, however, stirred up, so called sludge escape, due to the formation of turbulence behind the blades and thus a maximum efficiency cannot be reached. The load on the devices further leads to that they have to be manufactured in metal, and in order to prevent corrosion from eating the devices too fast they have to be manufactured in more acid resistant materials which increases costs.
It is previously known from SE-C-454,140 a device in settling basins and similar objects, for moving sludge and sediment gathered from the bottom of the basin, whereby a transporting device is arranged to be moved to and fro close to the bottom of the basin, whereby sludge and sediment is moved at the forward movement to one end of the basin, whereby the transporter consists of a mat comprising means having a wedge formed, triangular cross-section, which means are arranged essentially perpendicular to the moving direction of the transporter and the point of the wedge is directed in the returning direction and the wedge base of said means being directed in the forwarding direction.
DE-C-936,980 discloses a transporting element of a scraper wherein one embodiment shows a triangular cross-section, and one embodiment shows wingshaped cross-section, with a plane bottom surface, a substantially oblique convex surface, and a substantially vertical, broken surface, which latter is the transporting surface of sludge of the element.
The triangular form, a perpendicular triangle, has per se advantages by creating a very low resistance during the returning movement in the water. However, it has turned out that a certain sludge flight exists, particularly at the forward movement at the treatment of light sludge, when the sludge is pressed up along the base of the wedge, but also during the return movement when the vortices behind the base of the wedge become unfavourable.
Thus there is a demand for other types of transporting means for transporting sludge and sediment in settling basins. Hereby, one wants to increase the efficiency by minimizing sludge flight.
One object of the present invention is to obtain a device meeting these requirements.